Ion-sensing charge-accumulation circuits and methods

ABSTRACT

An ion-sensitive circuit can include a charge accumulation device, to accumulate a plurality of charge packets as a function of an ion concentration of a fluid, and at least one control and readout transistor, to generate an output signal as a function of the accumulated plurality of charge packets, the output signal representing the ion concentration of the solution. The charge accumulation device can include a first charge control electrode above a first electrode semiconductor region, an electrically floating gate structure above a gate semiconductor region and below an ion-sensitive passivation surface, a second charge control electrode above a second electrode semiconductor region, and a drain diffusion region. The first control electrode can control entry of charge into a gate semiconductor region in response to a first control signal. The ion-sensitive passivation surface can be configured to receive the fluid. The second charge control electrode can control transmission of the plurality of charge packets out of the gate semiconductor region and into the drain diffusion region in response to a second control signal. The drain diffusion region can receive the plurality of charge packets from the gate semiconductor region via the second electrode semiconductor region.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/964,938 filed Dec. 10, 2015, which is a U.S. application Ser. No.14/477,125 filed Sep. 4, 2014 (now U.S. Pat. No. 9,239,313), which is acontinuation of U.S. application Ser. No. 13/173,146 filed Jun. 30, 2011(now U.S. Pat. No. 8,858,782), which claims the benefit of U.S.Provisional Application No. 61/360,493 filed Jun. 30, 2010; 61/360,495filed Jul. 1, 2010; and 61/361,403, filed Jul. 3, 2010; each of which isherein incorporated by reference in their entirety.

BACKGROUND

Electronic devices and components have found numerous applications inchemistry and biology (more generally, “life sciences”), especially fordetection and measurement of various chemical and biological reactionsand identification, detection and measurement of various compounds. Onesuch electronic device is referred to as an ion-sensitive field effecttransistor, often denoted in the relevant literature as an “ISFET” (orpHFET). ISFETs conventionally have been explored, primarily in theacademic and research community, to facilitate measurement of thehydrogen ion concentration of a solution (commonly denoted as “pH”). Thechemically-sensitive sensor referred to herein may be implemented usingan ISFET, phFET, chemFet or some other transistor device capable ofperforming a similar function.

More specifically, an ISFET is an impedance transformation device thatoperates in a manner similar to that of a MOSFET (Metal OxideSemiconductor Field Effect Transistor), and is particularly configuredto selectively measure ion activity in a solution (e.g., hydrogen ionsin the solution are the “analytes”). A detailed theory of operation ofan ISFET is given in “Thirty years of ISFETOLOGY: what happened in thepast 30 years and what may happen in the next 30 years,” P. Bergveld,Sens. Actuators, 88 (2003), pp. 1-20 (“Bergveld”), which publication ishereby incorporated herein by reference in its entirety.

Details of fabricating an ISFET using a conventional CMOS (ComplementaryMetal Oxide Semiconductor) process may be found in Rothberg, et al.,U.S. Patent Publication No. 2010/0301398, Rothberg, et al., U.S. PatentPublication No. 2010/0282617, and Rothberg et al, U.S. PatentPublication 2009/0026082; these patent publications are collectivelyreferred to as “Rothberg”, and are all incorporated herein by referencein their entirety. In addition to CMOS, however, biCMOS (i.e., bipolarand CMOS) processing may also be used, such as a process that wouldinclude a PMOS FET array with bipolar structures on the periphery.Alternatively, other technologies may be employed wherein a sensingelement can be made with a three-terminal devices in which a sensed ionleads to the development of a signal that controls one of the threeterminals; such technologies may also include, for example, GaAs andcarbon nanotube technologies.

Taking a CMOS example, a P-type ISFET fabrication is based on a p-typesilicon substrate, in which an n-type well forming a transistor “body”is formed. Highly doped p-type (p+) regions S and D, constituting asource and a drain of the ISFET, are formed within the n-type well. Ahighly doped n-type (n+) region B may also be formed within the n-typewell to provide a conductive body (or “bulk”) connection to the n-typewell. An oxide layer may be disposed above the source, drain and bodyconnection regions, through which openings are made to provideelectrical connections (via electrical conductors) to these regions. Apolysilicon gate may be formed above the oxide layer at a location abovea region of the n-type well, between the source and the drain. Becauseit is disposed between the polysilicon gate and the transistor body(i.e., the n-type well), the oxide layer often is referred to as the“gate oxide.”

Like a MOSFET, the operation of an ISFET is based on the modulation ofcharge concentration (and thus channel conductance) caused by a MOS(Metal-Oxide-Semiconductor) capacitance. This capacitance is constitutedby a polysilicon gate, a gate oxide and a region of the well (e.g.,n-type well) between the source and the drain. When a negative voltageis applied across the gate and source regions, a channel is created atthe interface of the region and the gate oxide by depleting this area ofelectrons. For an n-well, the channel would be a p-channel (andvice-versa). In the case of an n-well, the p-channel would extendbetween the source and the drain, and electric current is conductedthrough the p-channel when the gate-source potential is negative enoughto attract holes from the source into the channel. The gate-sourcepotential at which the channel begins to conduct current is referred toas the transistor's threshold voltage VTH (the transistor conducts whenVGS has an absolute value greater than the threshold voltage VTH). Thesource is so named because it is the source of the charge carriers(holes for a p-channel) that flow through the channel; similarly, thedrain is where the charge carriers leave the channel.

As described in Rothberg, an ISFET may be fabricated with a floatinggate structure, formed by coupling a polysilicon gate to multiple metallayers disposed within one or more additional oxide layers disposedabove the gate oxide. The floating gate structure is so named because itis electrically isolated from other conductors associated with theISFET; namely, it is sandwiched between the gate oxide and a passivationlayer that is disposed over a metal layer (e.g., top metal layer) of thefloating gage.

As further described in Rothberg, the ISFET passivation layerconstitutes an ion-sensitive membrane that gives rise to theion-sensitivity of the device. The presence of analytes such as ions inan analyte solution (i.e., a solution containing analytes (includingions) of interest or being tested for the presence of analytes ofinterest), in contact with the passivation layer, particularly in asensitive area that may lie above the floating gate structure, altersthe electrical characteristics of the ISFET so as to modulate a currentflowing through the channel between the source and the drain of theISFET. The passivation layer may comprise any one of a variety ofdifferent materials to facilitate sensitivity to particular ions; forexample, passivation layers comprising silicon nitride or siliconoxynitride, as well as metal oxides such as silicon, aluminum ortantalum oxides, generally provide sensitivity to hydrogen ionconcentration (pH) in an analyte solution, whereas passivation layerscomprising polyvinyl chloride containing valinomycin provide sensitivityto potassium ion concentration in an analyte solution. Materialssuitable for passivation layers and sensitive to other ions such assodium, silver, iron, bromine, iodine, calcium, and nitrate, forexample, are known, and passivation layers may comprise variousmaterials (e.g., metal oxides, metal nitrides, metal oxynitrides).Regarding the chemical reactions at the analyte solution/passivationlayer interface, the surface of a given material employed for thepassivation layer of the ISFET may include chemical groups that maydonate protons to or accept protons from the analyte solution, leavingat any given time negatively charged, positively charged, and neutralsites on the surface of the passivation layer at the interface with theanalyte solution.

With respect to ion sensitivity, an electric potential difference,commonly referred to as a “surface potential,” arises at thesolid/liquid interface of the passivation layer and the analyte solutionas a function of the ion concentration in the sensitive area due to achemical reaction (e.g., usually involving the dissociation of oxidesurface groups by the ions in the analyte solution in proximity to thesensitive area). This surface potential in turn affects the thresholdvoltage of the ISFET; thus, it is the threshold voltage of the ISFETthat varies with changes in ion concentration in the analyte solution inproximity to the sensitive area. As described in Rothberg, since thethreshold voltage VTH of the ISFET is sensitive to ion concentration,the source voltage VS provides a signal that is directly related to theion concentration in the analyte solution in proximity to the sensitivearea of the ISFET.

Arrays of chemically-sensitive FETs (“chemFETs”), or more specificallyISFETs, may be used for monitoring reactions—including, for example,nucleic acid (e.g., DNA) sequencing reactions, based on monitoringanalytes present, generated or used during a reaction. More generally,arrays including large arrays of chemFETs may be employed to detect andmeasure static and/or dynamic amounts or concentrations of a variety ofanalytes (e.g., hydrogen ions, other ions, non-ionic molecules orcompounds, etc.) in a variety of chemical and/or biological processes(e.g., biological or chemical reactions, cell or tissue cultures ormonitoring, neural activity, nucleic acid sequencing, etc.) in whichvaluable information may be obtained based on such analyte measurements.Such chemFET arrays may be employed in methods that detect analytesand/or methods that monitor biological or chemical processes via changesin charge at the chemFET surface. Such use of ChemFET (or ISFET) arraysinvolves detection of analytes in solution and/or detection of change incharge bound to the chemFET surface (e.g. ISFET passivation layer).

Research concerning ISFET array fabrication is reported in thepublications “A large transistor-based sensor array chip for directextracellular imaging,” M. J. Milgrew, M. O. Riehle, and D. R. S.Cumming, Sensors and Actuators, B: Chemical, 111-112, (2005), pp.347-353, and “The development of scalable sensor arrays using standardCMOS technology,” M. J. Milgrew, P. A. Hammond, and D. R. S. Cumming,Sensors and Actuators, B: Chemical, 103, (2004), pp. 37-42, whichpublications are incorporated herein by reference and collectivelyreferred to hereafter as “Milgrew et al.” Descriptions of fabricatingand using ChemFET or ISFET arrays for chemical detection, includingdetection of ions in connection with DNA sequencing, are contained inRothberg. More specifically, Rothberg describes using a chemFET array(in particular ISFETs) for sequencing a nucleic acid involvingincorporating known nucleotides into a plurality of identical nucleicacids in a reaction chamber in contact with or capacitively coupled tochemFET, wherein the nucleic acids are bound to a single bead in thereaction chamber, and detecting a signal at the chemFET, whereindetection of the signal indicates release of one or more hydrogen ionsresulting from incorporation of the known nucleotide triphosphate intothe synthesized nucleic acid.

FIG. 1 is a cross-sectional diagram depicting an embodiment of aphysical structure of an ion-sensing system 20 that can include an ISFET24, a solution 28 provided at an ion-sensitive surface 32 of the ISFET24, and an electrode 36 in the solution 28 providing a reference voltageVREFA. The ISFET 24 can be fabricated in a CMOS process, and includen-type source and drain diffusion regions 40, 44 formed in a p-typesilicon substrate 48, source and drain connections 52, 56, a gate oxidelayer 60 over a channel-forming region 64 between the source and drain40, 44, a floating gate structure 68 formed over the gate oxide 60, anda passivation layer 72 formed over the gate structure 68. The floatinggate structure 68 can include a polysilicon gate 78 and one or moremetal layers 76 and via interconnections 80. The source and drainconnections 52, 54 can also include one or more metal layers 76 and viainterconnections 80. A dielectric isolation 84 can separate thesevarious structures.

In operation, the reference voltage VREFA can be supplied to theelectrode 36 in the solution 28, and the source and drains 40, 44 of theISFET 24 can be electrically connected to readout circuitry (not shown)through the source and drain connection structures 52, 56. The gate 78of the ISFET 24 may have no direct electrical connection to othercircuits, and thus can be an electrically floating structure. The effectof the ion concentration on the operation of the ISFET 24 can be modeledas a dependence of a threshold voltage of the ISFET 24 on the ionconcentration in the solution 28 resulting from an electrochemicalinteraction between the ion-sensitive passivation surface 32 of theISFET 24 and ions in the adjacent solution 28. The ion-sensing system 20can thus determine the ion concentration from a threshold voltagechange, itself determined by measuring a current or voltage change atthe source or drain 40, 44 of the ISFET 24, given a known referencevoltage VREFA and readout circuit type and operation.

However, problems exist with the ion-sensing system 20 of FIG. 1 and itsuse to detect the ion concentration of the solution 28. The change tothe voltage or current at the source or drain 40 ,44 of the ISFET 24,and thus in the readout circuit, induced by the change in thresholdvoltage can be small and difficult to accurately measure. Additionally,the threshold voltage itself can be a non-linear function of othervariables, such as the voltage across the source 40 to the substrate 48(i.e., body) of the ISFET 24. This can limit the linearity of thethreshold voltage calculations, if the source-to-body voltage is allowedto vary, or limit the type of readout circuit that can be used, if thesource-to-body voltage is to be maintained relatively constant topreserve linearity. In a similar vein, both dynamic range andsignal-to-noise performance of the ion-sensing system 20 of FIG. 1 areconcerns. Overall, these concerns can necessitate the use of complex,and thus space-consuming and costly, readout circuits, or limit theperformance metrics that can be achieved using this ion-sensing system20.

Thus, a need exists for a way to achieve improved speed, accuracy,linearity and other performance metrics when measuring ion concentrationin a solution, but while still utilizing devices that can bemanufactured in bulk using a CMOS process.

BRIEF DESCRIPTION OF THE DRAWINGS

So that features of the present invention can be understood, a number ofdrawings are described below. However, the appended drawings illustrateonly particular embodiments of the invention and are therefore not to beconsidered limiting of its scope, for the invention may encompass otherequally effective embodiments.

FIG. 1 is a cross-sectional diagram depicting an embodiment of anion-sensing system including an ion-sensitive field effect transistor.

FIG. 2 is a partially cross-sectional, partially schematic diagram of anion-sensing system including a ion-sensitive charge accumulation device.

FIGS. 3A-3D are cross-sectional, potential and charge diagrams depictingan embodiment of an operational cycle of the ion-sensitive chargeaccumulation device.

FIG. 4 is a signal diagram depicting an embodiment of control signalsfor controlling the operation of the charge accumulation device.

FIG. 5 is a schematic diagram depicting an embodiment of anion-sensitive pixel array having a plurality of pixel circuits, eachincluding the charge accumulation device and control and readouttransistors.

FIG. 6 is a partially cross-sectional, partially schematic diagramdepicting an embodiment of a pixel circuit including the chargeaccumulation device and control and readout transistors.

FIG. 7 is a partially cross-sectional, partially schematic diagramdepicting another embodiment of a pixel circuit including the chargeaccumulation device and control and readout transistors.

FIGS. 8A-8D are cross-sectional, potential and charge diagrams depictingembodiments of operation of the pixel circuit depicted in FIG. 7.

FIG. 9 is a partially cross-sectional, partially schematic diagramdepicting another embodiment of a pixel circuit including the chargeaccumulation device and control and readout transistors.

FIG. 10 is a partially cross-sectional, partially schematic diagramdepicting another embodiment of a pixel circuit including the chargeaccumulation device and control and readout transistors.

FIGS. 11A-11D are cross-sectional, potential and charge diagramsdepicting embodiments of operation of the pixel circuit depicted in FIG.10.

DETAILED DESCRIPTION

An ion-sensitive circuit can include a charge accumulation device, toaccumulate a plurality of charge packets as a function of an ionconcentration of a fluid, and at least one control and readouttransistor, to generate an output signal as a function of theaccumulated plurality of charge packets, the output signal representingthe ion concentration of the solution. The charge accumulation devicecan include a first charge control electrode above a first electrodesemiconductor region, an electrically floating gate structure above agate semiconductor region and below an ion-sensitive passivationsurface, a second charge control electrode above a second electrodesemiconductor region, and a drain diffusion region. The first controlelectrode can control entry of charge into a gate semiconductor regionin response to a first control signal. The ion-sensitive passivationsurface can be configured to receive the fluid. The second chargecontrol electrode can control transmission of the plurality of chargepackets out of the gate semiconductor region and into the draindiffusion region in response to a second control signal. The draindiffusion region can receive the plurality of charge packets from thegate semiconductor region via the second electrode semiconductor region.

The ion-sensitive circuit can operate according to a method to detectthe ion concentration of the fluid. The method can include (i) passingthe fluid having the ion concentration over the ion-sensitivepassivation surface of the charge accumulation device; (ii) individuallyforming, one at a time, the plurality of charge packets in the gatesemiconductor region of the charge accumulation device as a function ofthe ion concentration in the fluid; (iii) applying a control signal tothe second control electrode of the charge accumulation device tocontrol transmission of the charge packets one at a time from the gatesemiconductor region; (iv) accumulating the plurality of charge packetsat the drain diffusion region of the charge accumulation device at aselectable charge packet accumulation frequency; and (v) generating anoutput signal using the at least one control and readout transistor as afunction of the accumulated plurality of charge packets at the drainregion at a selectable output signal generation frequency, wherein theoutput signal represents a measure of the ion concentration in thefluid.

FIG. 2 depicts a partially cross-sectional, partially schematic view ofan embodiment of an ion-sensing system 100, having an ion-sensitivecharge-accumulation device 104, that can detect ion concentration in asolution 108 to perform DNA sequencing and other applications. The ionsensing system 100 can include the charge-accumulation device 104, acontrol and readout circuit 112, an electrode 116, the solution 108 anda micro-particle or micro-bead 120. In FIG. 2, the charge-accumulationdevice 104, electrode 116, solution 108 and micro-particle 120 aredepicted in a cross-sectional representation, while the control circuit112, and its connection to terminals of the charge-accumulation device104 are depicted schematically. For convenience, the schematic portionsof FIG. 2 are indicated by dashed lines.

The charge accumulation device 104 can be fabricated in a CMOS processand be part of an integrated circuit including the charge-accumulationdevice 104 and the control and readout circuit 112. N-type source anddrain diffusion regions 124, 128 can be formed in a p-type epitaxiallayer 132 formed on a p-type silicon substrate 136. The source and draindiffusion regions 124, 128 can have lightly-doped portions 140, 144 withrelatively less doping than other portions of the drain and diffusionregions 124, 128. Source and drain connections 148, 152 can include oneor more metal layers 156 and via interconnections 160. A gate oxidelayer 164 can be formed over a region 168 of the p-type epitaxial layer132 between the source and drain 124, 128, and a floating gate structure172 and first and second control electrode structures 176, 180 canformed over the gate oxide 164. The gate oxide 164 can include amaterial such as silicon dioxide. The floating gate structure 172 caninclude a n-type polysilicon gate 184 and one or more metal layers 156and via interconnections 160. The control electrode structures 176, 180can also include first and second n-type polysilicon electrodes 186,188, respectively, and one or more metal layers 156 and viainterconnections 160. The metal layers 156 and vias 160 can each includeone or more layers of conductive materials such as tungsten, aluminum,copper, titanium, and nitrides and silicides thereof. Dielectricisolation 192 between these various structures can include one or morelayer of silicon oxide, borophosphosilicate glass, or combinationsthereof. A passivation layer 196 can be formed over the gate structure172 and dielectric isolation 196, and include a well 200 having anion-sensitive surface 204 to accommodate the micro-particle 120 (ormicro-bead 120), which can have a plurality of replicated DNA strandsattached to its surface. The passivation layer 196 can include amaterial such as silicon nitride, silicon oxynitride, and polyimide. Asubstrate connection diffusion region 208 and can have an associatedconnection structure 212 similar to the drain and source connectionstructures 148, 152.

Reference to diffusion regions 124, 128 as source and drain diffusionregions 124, 128 is for convenience of reference herein. These diffusionregions 124, 128, however, do not need to have all the same propertiesas a source and drain of a transistor. Instead, the source and draindiffusion regions 124, 128 can also be referred to as simply diffusionregions 124, 128, without necessarily implying transistor source anddrain functionality. In some instances, however, as is apparent from thedescription herein, the diffusion regions 124, 128 may indeed haveproperties like those of a source and drain of a transistor.

The ion-sensing system 100 can be used to perform DNA sequencing orother applications involving the sensing of ions in the solution 108. Aplurality of the DNA-strand-bearing micro-particles 120 can beintroduced to a surface of an integrated circuit containing a pluralityof the charge-accumulation devices 104 depicted in FIG. 2, with themicro-particles 120 settling into the wells 200 of the passivation layer196. A plurality of different solutions 108 can be sequentiallyintroduced to and received by the surface of the integrated circuit,including in the wells 200 in the passivation layer 196. Each of thesolutions 108 in the sequential series of solutions can include adifferent nucleotide or nucleobase. Each solution, or reagent, may ormay not react with the DNA strands attached to the micro-particle 120,depending on whether the solution's particular nucleotide or nucleobasematches or compliments a currently reactive position along the sequenceof nucleobases of the attached DNA strands. When a particular solution108 in the sequence of administered solutions does react with the DNAstrands, a plurality of ions, such as, e.g., protons (e.g., hydrogenH+ions), pyrophosphate (i.e., PPi), or both, may be released. Theion-sensitive charge-accumulation device 104 can detect theconcentration of ions in the solution 108, according to principles ofits operation discussed herein, to provide data as to the identity ofthe nucleobase of the currently reactive position of the attached DNAstrands, thereby providing data to sequence the strands.

In operation, the ion-sensitive charge-accumulation device 104 cangenerate and accumulate a plurality of charge packets 216 (shown, e.g.,in FIGS. 3A-3D) as a function of and in response to ion concentrationsin the solution 108 provided to the ion-sensitive surface 204 of thewell 200 in the passivation layer 196, using potential energy barriersand gradients created through the application of control signals to thesource 124, drain 128, first and second control electrodes 186, 188 bythe control and readout circuitry 112. FIGS. 3A-D depict embodiments ofa plurality of cycles of operation of the charge-accumulation device 104to generate and accumulate charge packets in response to ionconcentrations in the solution 108. Each of FIGS. 3A-3D can depict acomplete cycle of an embodiment of operation of the charge accumulationdevice 104. The top of each of FIGS. 3A-3D depicts a simplifiedpartially cross-sectional, partially schematic representation of thecharge-accumulation device 104, showing only the source and drainregions 124, 128, the gate oxide 164, the floating gate 184, and thefirst and second control electrodes 186, 188. For convenience ofillustration and explanation, other components of thecharge-accumulation device 104 are omitted from the depictions, althoughomitted components can be present in actual embodiments. In each figure,below the representation of the charge-accumulation device 14 are fourdiagrams depicting potential energies and charge present in the chargeaccumulation device 104 at different stages during the operational cycleof the device 104. Each of these diagrams is aligned to and depicts thepotential energy and charge in specific spatial regions of the chargeaccumulation device 104 depicted at the top of the figure, so that eachdiagram therefore depicts the potential energy and charge in the n-typesource and drain regions 124, 128 and in the p-type epitaxial regionsunder the control electrodes 186, 188 and the floating gate 184.

FIG. 4 is a signal diagram depicting an embodiment of a plurality ofcontrol signals that can be used to control the operation of the chargeaccumulation device 104 depicted in FIGS. 3A-3D. In FIG. 4, controlsignals VS, VD, VC2 delivered to the source 124, drain 128 and secondcontrol electrode 188 can vary between voltage values representing logiclow and logic high states, such as lower and upper power supplyvoltages. Another control signal VC1 (not shown) can be delivered to thefirst control electrode 186, and may not represent a logic state andinstead may assume an intermediate voltage value, such as a valuebetween lower and upper power supply voltages. Other embodiments ofcontrol signals are possible, such as various combinations of controlsignals that vary between voltage values representing logic low andlogic high states (such as lower and upper power supply voltages) andcontrol signals that do not represent logic states and instead assumeintermediate voltage values (such as a value between lower and upperpower supply voltages). Embodiments of control signals can also be afunction of specific embodiments of the physical structure of thecharge-accumulation device 104. For example, the height of the gateoxide 164 between the first control electrode 186, floating gate 184,and second control electrode 188 can be selected to control the relativeimpact on potential energy in the charge accumulation device 104 of agiven control voltage delivered to the first control electrode 186,floating gate 184 and second control electrode 188, respectively.

Prior to a first phase of the operational cycle depicted in FIG. 3A, ina pre-cycle reset phase (not shown in FIG. 3A), any charge remaining inthe charge accumulation device 104 from a previous operation of thedevice 104 can be removed by delivering suitable control signals to thesource 124, drain 128 and control electrodes 186, 188. In the embodimentdepicted in FIG. 4, a high voltage can be delivered to the drain 124 andsecond control electrode 188, a low voltage to the source 124, and anintermediate voltage to the first control electrode 186 to clear anycharge from a previous operation of the device 104.

In a first phase 220 of the operational cycle depicted in FIG. 3A, anycharge packet 216 formed in the semiconductor region 224, e.g., p-typeepitaxial region 224, under the floating gate 184 can be transferred tothe drain diffusion region 208. This can be done by creating a potentialgradient in the semiconductor regions 228, 224, 232 under the firstcontrol electrode 186, gate 184, and second control electrode 188 todirect the charge under the gate 184 to move to the drain diffusionregion 208. Specifically, a relatively lower potential, and thus arelatively higher barrier to electron flow, can be created in the region228 under the first control electrode 186, and a relatively higherpotential, and thus a relatively lower barrier to electron flow, can becreated in the region 232 under the second control electrode 188.

The potential diagrams of FIGS. 3A-3D, and other potential diagramsdepicted and discussed herein, follow the convention that increasingpotential energies correspond to downward directions in the potentialenergy diagrams. Thus, relatively higher potential energies are depictedby spatially lower indications in the diagrams. Relatively higherpotential energies can be induced in p-type regions by applyingrelatively higher voltages to the control electrodes 186, 188 or suchmanifesting on the floating gate 184, and can represent a lowerpotential barrier to the accumulation or passage of electrons in p-typeregions. By contrast, relatively lower potential energies can be inducedin p-type regions by relatively lower voltages applied to the controlelectrodes 186, 188 or such manifesting on the floating gate 184, andcan represent a higher potential barrier to the accumulation or passageof electrons in p-type regions.

Returning to FIG. 3A, the first stage potential energy gradient can becreated in the regions 228, 224, 232 under the first control electrode186, floating gate 184 and second control electrode 188 by applicationor manifestation of increasing voltages to the first control electrode186, floating gate 184 and second control electrode 188, respectively,assuming a uniform gate oxide thickness. Alternatively, this potentialenergy gradient can also be created with other applied or manifestedvoltages and a non-uniform gate oxide thickness. FIG. 4 depicts anembodiment in which a high voltage can be delivered to the secondcontrol electrode 188 and source 124, a low voltage can be delivered tothe drain 124, and the intermediate voltage to the first controlelectrode 186 in the first phase. Other embodiments of first phasecontrol signals are possible.

Note that, because FIG. 3A depicts the first of several cycles ofoperation of the charge accumulation device 104 after a pre-cycle resetphase has removed any remaining charge packets 216 from a previousoperation, no charge packet 216 has yet developed under the gate 184,and thus no charge packet 216 is transferred to the drain 128 in thefirst phase 220 depicted in FIG. 3A. However, the transfer during thefirst phase 220 of the charge packet 216 from under the gate 184 to thedrain region 128 can be illustrated by the succeeding cycles depicted inFIGS. 3B-3D.

In a second phase 236 of the operational cycle depicted in FIG. 3A, thepotential barrier under the second control electrode 188 can be raisedto prevent charge packet flow from the region 224 under the gate 184through the region 232 under the second electrode 188 to the drainregion 128. FIG. 4 depicts an embodiment of second phase control signalsin which a low voltage can be delivered to the second control electrode188 and drain 128, a high voltage to the source 124, and theintermediate voltage to the first control electrode 186. Other secondphase control signal embodiments are possible.

In a third phase 240 of the operational cycle depicted in FIG. 3A,charge, e.g., electrons, can be injected into the p-type region 224under the floating gate 184 from the n-type source region 124. Becausethe potential barrier under the second electrode 188 has been raisedduring the second phase 236 to be higher than under the first controlelectrode 186 and floating gate 184, the charge 242 can flood the chargeaccumulation device 104 from the source 124 to under the first controlelectrode 186 and the floating gate 184, but not under the secondcontrol electrode 188. Charge 242 can be injected into the chargeaccumulation device 104 through the source 124 in various ways, such asthrough the use of a current source or charge pump selectively injectingcharge into the source 124, or by applying a suitable voltage to thesource 124 in the context of voltages supplied to or manifestingelsewhere in the charge accumulation device 104, e.g., on the firstcontrol electrode 186 and floating gate 184. FIG. 4 depicts anembodiment of the control signals in which a low voltage can bedelivered to the source 124, while relatively higher voltages can bedelivered or manifest on the first control electrode 186 and floatinggate 184 (e.g., the intermediate voltage delivered to the first controlelectrode 186 and the potential induced on the floating gate 184 by theion concentration in the solution 108).

In a fourth phase 244, the charge injection from the source 124 into thecharge accumulation device 104 can be ended, and excess charge in thesource 124 and under the first control electrode 186 withdrawn. However,because a potential differential can exist between under the firstcontrol electrode 186 and under the floating gate 184, a packet ofcharge 216 can remain under the floating gate 184 as a result of thispotential difference. This potential difference can be a function of thevoltage VC1 applied to the first control electrode 186 and the voltagemanifesting on the floating gate 184 due to the ion concentration in thesolution 104. Thus, the size of the charge packet 216 remaining underthe floating gate 184 in the fourth phase 244 can be a function of thevoltage VC1 applied to the first control electrode 186 and the voltagemanifesting on the floating gate 184 due to the ion concentration in thesolution 108, and thus a measure of the ion concentration in thesolution 108, given a known voltage VC1 applied to the first controlelectrode 186.

Finally, in a first phase 220 of the next cycle of operation of thecharge accumulation device 104, depicted in FIG. 3B, and as discussedabove in regard to the first phase 220 of the first cycle of operationdepicted in FIG. 3A, the charge packet 216 remaining under the floatinggate 184 after the fourth stage 244 of the first cycle of operation canbe transferred from under the gate 184, through the region 232 under thesecond control electrode 188, and into the drain diffusion region 128,where it can be maintained. Thus, after a single cycle of operation, asingle charge packet 216, having a size that can be a measure of the ionconcentration of the solution 108, can have been collected at the drain128 of the charge accumulation device 104.

Although the charge accumulation device 104 can provide a measure of theion concentration in the solution 104 with just a single charge packet216 collected at the drain 128, the charge accumulation device 104 canalso detect and provide a measure of the ion concentration in thesolution 108 by collecting a plurality of charge packets 216 at thedrain 128 over a plurality of cycles of operation of the chargeaccumulation device 104. The collected plurality of charge packets 216can also provide a measure of the ion concentration in the solution 108.FIGS. 3B-3D depict second, third and fourth cycles of operation of thecharge accumulation device 104 following the first cycle of operationdepicted in FIG. 3A. After each cycle, another charge packet 216 can beaccumulated at the drain 128 as a function of the ion concentration. InFIG. 3B, in a first phase 220 of the second cycle, a first charge packet216 can be trapped at the drain 128 after the first cycle depicted inFIG. 3A. In FIG. 3C, in a first phase 220 of the third cycle, first andsecond charge packets 216 b can be accumulated at the drain 128following the second cycle depicted in FIG. 3B. In FIG. 3D, in a firstphase 220 of the fourth cycle, first, second and third charge packets216 c can be accumulated at the drain 128 following the third cycledepicted in FIG. 3C.

Providing a measure of the ion concentration in the form of anaccumulated plurality of charge packets 216 can provide a measure havingan increased signal level and signal-to-noise ratio, improvedperformance matching of a plurality of charge accumulation devices 104,and reduced flicker noise, in comparison to both providing a measure ofthe ion concentration in the form of only a single charge packet 216 andin the form of a threshold voltage variation, as discussed above inregard to FIG. 1. For example, due to the thermal activity of chargessuch as holes and electrons, generated charge packets are typicallyassociated with a degree of noise. In one embodiment, the accumulationof a single charge packet may be accompanied by a noise charge errorproportional to k·T·C, where k is Boltzmann's constant, T is thetemperature in Kelvins, and C is the capacitance of the charge gatheringregion under the floating gate. When the charges are electrons, thevoltage noise error for each collected packet can be equal to (squareroot of (k·T·C))/q, where q is the charge of an electron, and C is equalto the oxide capacitance Cox multiplied by the area of the floating gate184. However, the signal-to-noise ratio SNR of an output signal based onthe accumulation of a plurality of charge packets can be proportional tothe square root of (2·n·C/k·T), where n is the number of packets 216accumulated, i.e., the number of operational cycles of the chargeaccumulation device 104 utilized to provide a single ion concentrationmeasurement. Thus, the signal-to-noise ratio of the ion concentrationmeasurement can increase in proportion to the square root of the numberof packets 216 in a single measurement.

After accumulation at the drain 128 of the charge accumulation device104, the plurality of charge packets 216 can be converted to an outputsignal or transferred out of the charge accumulation device usingembodiments of the control and readout circuitry 112. The control andreadout circuit 112 can control various aspects of both the cyclicaloperation of the charge accumulation device 104 and generation of anoutput signal based on the accumulated charge packets 216. The controland readout circuit 112 can control the charge accumulation device 104by providing control signals to the source 124, drain 128, first controlelectrode 186 and second control electrode 188 to accumulate packets 216at a selectable predetermined accumulation frequency. The accumulationfrequency can be the frequency of operation of a single operationalcycle of the charge accumulation device 104. The accumulation frequencycan be selected based on or as a function of the anticipated rate ofchange of the ion concentration in the solution, the performancecharacteristics of the charge accumulation device 104 and control andreadout circuitry 112, or a combination thereof. The control and readoutcircuit 112 can also control the generation of an output signal based onthe charge packets 216 accumulated by the charge accumulation device 104by providing control signals thereto to generate an output signal ortransfer accumulated pluralities of charge packets 216 out of the chargeaccumulation device 104 at a selectable output generation frequency. Theoutput generation frequency can be the frequency of generation of asingle output signal value from the accumulated packets 216 in thecharge accumulation device 104. The output generation frequency can beselected to be based on or be a function of the anticipated rate ofchange of the ion concentration in the solution 108, the performancecharacteristics of the charge accumulation device 104 and the controland readout circuitry 112, or a combination thereof. When the outputsignal is based on a plurality of accumulated charge packets 216, theoutput generation frequency can be less than the charge packetaccumulation frequency.

A single charge accumulation device 104, along with associated portionof the control and readout circuitry 112 dedicated to that device 104,can represent a single ion-sensing pixel 246 in an array 250 of aplurality of ion-sensing pixels 246. FIG. 5 depicts an embodiment of anion-sensing pixel array 250 having a plurality of ion-sensing pixels246. Each pixel circuit 246 can include a charge accumulation device 104and an associated portion of the control and readout circuit 112. Theassociated portion of the control and readout circuit 112 can be part ofand dedicated to the particular pixel 246. The pixel array can bearranged into a plurality of rows and columns of pixels 246. The pixelarray 250 can be controlled, addressed and have data input and output bya plurality of row and column control, addressing and data lines,including one or more row control, addressing and data lines R1-Rx,collectively referred to as row lines R1-Rx, and one or more columncontrol, addressing and data lines C1-Cx, collectively referred to ascolumn lines C1-Cx, for each row and each column.

FIG. 6 depicts an embodiment of an ion-sensing pixel 246 a havingembodiments of the charge accumulation device 104 and associated controland readout circuit portion 112. Note that FIG. 6 again depicts asimplified representation of the charge accumulation device 104, howeverembodiments of the depicted pixel 246 a can include other components ofthe charge accumulation device 104, such as the additional componentsdepicted in FIG. 2. The embodiment of FIG. 6 can be referred to as athree transistor, three electrode, or 3T3E, pixel 246. The control andreadout circuit 112 can include three transistors, including a resettransistor M1 and a pair of readout transistors M2, M3. The chargeaccumulation device 104 can have three electrodes, including the firstand second control electrodes 186, 188 and the floating gate 184. Thecontrol and readout circuit 112 can receive a plurality of column lines,including first and second column lines C1A, C1B, and a plurality of rowlines, including first and second row lines R1A, R1B.

For purposes of discussion with reference to sources and drains, etc.,it will be assumed that the control and readout transistors M1, M2, M3in the pixel embodiment 246 a of FIG. 6, and in other pixel embodimentdepictions discussed herein, are NMOS transistors. However, in otherpixel embodiments, the control and readout transistors can be eitherNMOS or PMOS transistors, or any combination thereof. In FIG. 6, thefirst readout transistor M2 can have a gate connected to the drainregion 128 of the charge accumulation device 104, a source connected toa drain of the second readout transistor M3, and a drain connected tothe second column line C1B. The second readout transistor M3 can have agate connected to the first row line R1A, a source connected to thefirst column line C1A, and a drain connected to the source of the fistreadout transistor M2. The reset transistor M1 can have a sourceconnected to the drain 128 of the charge accumulation device 104, a gateconnected to the second row line R1B, and a drain connected to thesecond column line C1B.

The embodiment of the control and readout circuit 112 of FIG. 6 can beused to control the charge accumulation device 104 and generate anoutput signal in conjunction with the operation of the chargeaccumulation device 104 depicted in FIGS. 3A-3D and FIG. 4. In a resetoperation, the reset transistor M1 and the second row and column linesR1B, C1B can be used to reset the charge accumulation device 104 in thepre-cycle reset phase, as discussed above. For example, a logic highsignal, or high-valued voltage such as an upper power supply, can bedelivered to the second row and column lines R1B, C1B while a logic lowsignal, or low-valued voltage such as a low power supply, can bedelivered to the first row line R1A, to turn on the rest transistor M1while turning off the second readout transistor M3. This can deliver ahigh voltage value to the drain region 128 of the charge accumulationdevice 104, resulting in the clearing out of any charge packets 216accumulated at the drain 128, thus resetting the charge accumulationdevice 104.

In a readout operation, the readout transistors M2, M3 and the first rowand column lines R1A, C1A can be used to generate an output signal as afunction of the charge packets 216 accumulated at the drain region 128of the charge accumulation device 104 after one or more cycles ofoperation of the device 104, as discussed above. For example, a logichigh signal or high-valued voltage such as an upper power supply, oralternatively an intermediate-valued voltage such as a voltage betweenthe upper and lower power supplies, can be delivered to the first rowline R1A while a logic low signal, or low-valued voltage such as a lowpower supply, can be delivered to the second row line R1B, essentiallyactivating the first and second readout transistors M2, M3 while turningoff the reset transistor M1. In this mode, the first and second readouttransistors M2, M3 can act as an amplifier to convert the one or morecharge packets 216 accumulated at the drain region 128 of the chargeaccumulation device 104 to a voltage output signal on the first columnline C1A. The first readout transistor M2 can be modeled as operating ina source follower configuration, receiving an input voltage from thedrain 128 of the charge accumulation device 104 and providing anintermediate output voltage at its source to the drain of the secondreadout transistor M3. The conversion of the charge packets 216 to aninput voltage at the drain 128 of the charge accumulation device 104 canbe an inherent result of the accumulation of charge at the drain 128.The second readout transistor M3 can be modeled as operating in acascode configuration with respect to the source follower first readouttransistor configuration, thereby boosting the gain of the twotransistor combination in comparison to the source follower alone. Thesecond readout transistor M3 can receive the intermediate output fromthe source of the source follower and provide the output voltage to thefirst column line C1A at this source.

FIG. 7 depicts another embodiment of an ion-sensing pixel 246 b havingembodiments of the charge accumulation device 104 and associated controland readout circuit portion 112. Note that FIG. 7 again depicts asimplified representation of the charge accumulation device 104, howeverembodiments of the depicted pixel 246 b can include other components ofthe charge accumulation device 104, such as the additional componentsdepicted in FIG. 2. The embodiment of FIG. 7 can be referred to as a twotransistor, three electrode, or 2T3E, pixel. The control and readoutcircuit 112 can include two transistors, including a reset transistor M4and a readout transistor M5, and the charge accumulation device 104 canhave three electrodes, including the first and second control electrodes186, 188 and the floating gate 184. The control and readout circuit 112can receive a row line R2A and a plurality of column lines, includingfirst and second column lines C2A, C2B.

The pixel embodiment 246 b of FIG. 7 can reduce the number of readouttransistors, in comparison to the pixel embodiment 246 a of FIG. 6, bymanagement of the voltage at the drain 128 of the charge accumulationdevice 104 to eliminate the need for a row select functionality servedby the first row line R1A of the pixel embodiment 246 a of FIG. 6. Thedrain 128 in each of FIGS. 7 and 9, also called a floating diffusion128, can have the property of a capacitance. The row select device canbe eliminated by managing the floating diffusion potential when thepixel 246 b is not active. The gate tied to the floating diffusion 128,for example M5 in FIG. 7, can remain at a low potential in order todisable the source follower M5 when not in use. Therefore, after thepixel 246 b has been read out, the floating diffusion 128 can be sampledto a low potential and then left at that potential until the pixel 246 bis read again. A reason the potential stays low can be that the floatingdiffusion 128 acts like a capacitor that holds the low voltage sampledonto it.

In FIG. 7, note that the depicted spatial arrangement of the first andsecond control electrodes 186, 188 is reversed from that shown inprevious figures, with the first control electrode 186 depicted on theright-hand side and the second control electrode 188 depicted on theleft-hand side. Again, for purposes of discussion with reference tosources and drains, etc., it will be assumed that the control andreadout transistors are NMOS transistors, however the control andreadout transistors in any of the pixel embodiments can be either NMOSor PMOS transistors, or any combination thereof. The reset transistor M4can have a source connected to the drain 128 of the charge accumulationdevice 104, a gate connected to the row line R2A, and a drain connectedto the second column line C2B. The readout transistor M5 can have a gateconnected to the drain region 128 of the charge accumulation device 104(and thus also the source of the reset transistor M4), a sourceconnected to the source 124 of the charge accumulation device 104 andthe first column line C2A, and a drain connected to the second columnline C2B.

FIGS. 8A-8D depict an embodiment of the operation of the pixelembodiment 246 b of FIG. 7 to accumulate a plurality of charge packets216 as a function of the solution ion concentration and generate anoutput signal as a function of the accumulated charge packets 216. FIGS.8A-8D depict a plurality of complete cycles of an embodiment ofoperation of the charge accumulation device 104. Similarly to FIGS.3A-3D, the top of each of FIGS. 8A-8D depicts a simplified partiallycross-sectional, partially schematic representation of the pixel 246,including the charge-accumulation device 104 and control and readouttransistors M4, M5. For convenience of illustration and explanation,other components of the charge-accumulation device 104 and control andreadout transistors M4, M5 are omitted from the depictions, althoughomitted components can be present in actual embodiments. In each figure,below the representation of the charge-accumulation device 104 andcontrol and readout transistors M4, M5 are four diagrams depicting, in amanner spatially aligned to the pixel representation above, potentialenergies and charge accumulation in the charge accumulation device 104and control and readout transistors M4, M5 at different phases of anoperational cycle of the pixel 246 b.

In a reset phase 254, the reset transistor M4 and the row and secondcolumn line R2A, C2B can be used to reset the charge accumulation device104. A logic high signal, or high-valued voltage such as an upper powersupply, can be delivered to the row and second column lines R2A to turnon both the rest transistor M4 and the readout transistor M5. This candeliver a high voltage value to both the drain 128 and source 124 of thecharge accumulation device 104, resulting in the clearing of any chargeaccumulated at the drain and source 128, 124 of the charge accumulationdevice 104, and thus resetting the device 104.

In a first phase 258 of the operational cycle, the reset transistor M4can be turned off and a potential barrier raised (i.e., a low potentialcreated) under the second electrode 188, preparing the chargeaccumulation device 104 for generating a charge packet 216 under thefloating gate 184. The reset transistor M4 can be turned off and the lowpotential under the second electrode 188 created by delivering a logiclow or low-valued voltage to the row line R2A and second controlelectrode 188. Note that at the end of the first phase 258, a highpotential, and thus a low potential barrier, is left existing under thegate of the readout transistor M4.

In a second stage 262 of the operational cycle, charge, e.g., electrons,can be injected into the p-type region 224 under the floating gate 184from the n-type source region 124 of the charge-accumulation device 104.The charge 264 can flood the charge accumulation device 104 from thesource 124 to under the floating gate 184, but not under the secondcontrol electrode 188. Charge can be injected into the chargeaccumulation device 104 through the source 124 in various ways, such asthrough the use of a current source or charge pump selectively injectingcharge into the source 124, or by applying a suitable voltage to thesource 124 in the context of voltages supplied to or manifesting on thefirst control electrode 186 and floating gate 184, such as, e.g., alogic-low or low-valued voltage. The charge can be injected under thefloating gate 184 through the source 124 using the first column lineC2A, and thus charge 265 can simultaneously be injected into the readouttransistor M4, including both its source and drain regions and under itsgate, due to the low potential barrier left existing under the gate ofthis transistor M4 after the first phase 258 of the operational cycle.

In a third stage 266, the charge injection into the charge accumulationdevice and readout transistor can be ended, and excess charge in thesedevices withdrawn. However, because a potential differential can existbetween under the first control electrode 186 and under the floatinggate 184, a charge packet 216 can remain under the floating gate 184 asa function of this potential difference. As discussed above in regard toFIGS. 3A-3D, the size of the charge packet 216 remaining under thefloating gate 184 in this stage 266 can be a function of the voltageapplied to the first control electrode 186 and the voltage manifestingon the floating gate 184 due to the ion concentration in the solution108, and thus be a measure of the ion concentration in the solution 108,given a known voltage applied to the first control electrode 186.

Finally, in a last phase 270 of the first operational cycle, depicted inFIG. 8B, the charge packet 216 remaining under the floating gate 184after the third stage 266 of the cycle can be transferred from under thegate 184, through the region 232 under the second control electrode 188,and into the drain diffusion region 128, where it can be maintained,similar to as discussed above in regard to the first phase 220 of theoperational cycle of FIGS. 3A-3D. The charge packet 216 can betransferred to the drain region 128 by lowering the potential barrierunder the second control electrode 188 by delivering a logic high orhigh-valued voltage to the second control electrode 188. The chargedelivered to the drain 128 of the charge accumulation device 104incrementally decreases the voltage at this drain 128, and because thegate of the readout transistor M5 is connected to the drain 128, thecharge packet 216 thus also incrementally increases the voltage at thegate of the readout transistor M5, as evidenced by the incrementallydecreased potential barrier 272 under the gate of this readouttransistor M5 at the end of the last stage 270 of the operational cycle.In this way, the voltage at and the potential under the gate of thereadout transistor M5 can be a function of the charge packets 216accumulated at the drain 128 of the charge accumulation device 104 atthe end of every operational cycle.

FIGS. 8B-8C depict a second operational cycle, in which another chargepacket is accumulated as in the first operational cycle depicted inFIGS. 8A-8B, with the result being the voltage at and the potentialunder the gate of the readout transistor again tracking the size of theplurality of accumulated charge packets 216. FIG. 8C also depictsanother reset phase after the second operational cycle.

An output can be generated by sampling the voltage at the source of thereadout transistor M5. This can occur during a readout phase 274. Theoutput can also be generated using a double sampling technique in whichboth a first output and a second output is provided. The first outputcan represent an output produced by a background level of charge presentat the drain 128 of the charge accumulation device 104. The secondoutput can represent an output produced by a both the background levelof charge and the plurality of charge packets 216 accumulated at thedrain 128. Subtracting the first output from the second output can thusproduce a more accurate measurement of the charge packets 216accumulated to represent the ion concentration of the solution 108. Inone embodiment, the first output can be sampled during the first phase258 of the operational cycle. The second the second output can then bethat sampled during the readout phase 274.

FIG. 8D also depicts an embodiment of a deactivation sequence for a rowof the pixels of the embodiment 246 a depicted in FIG. 7. In a firstphase 278 of the deactivation sequence, a logic low or low-valuedvoltage is delivered to the row line R2A, while a logic high or highvalued voltage is delivered to the first column line C2A. In a secondphase 282 of the deactivation sequence, a logic high or high-valuedvoltage is delivered to the row line R2A, while the a logic low or lowvalued voltage is delivered to the first column line C2A. In a thirdphase 286 of the deactivation sequence, a logic low or low-valuedvoltage is delivered to the row line R2A, while the logic low or lowvalued voltage is maintained at the first column line C2A. In a fourthphase 290 of the deactivation sequence, a logic low or low-valuedvoltage is maintained on the row line R2A, while a logic high or highvalued voltage is delivered to the first column line C2A.

FIG. 9 depicts another embodiment 246 c of a two transistor, threeelectrode, or 2T3E, pixel. The control and readout circuit 112 caninclude two transistors, including a reset transistor M6 and a readouttransistor M7, and the charge accumulation device 104 can have threeelectrodes, including the first and second control electrodes 186, 188and the floating gate 184. The control and readout circuit 112 canreceive a row line R3A and a plurality of column lines, including firstand second column lines C3A, C3B. The reset transistor M6 can have asource connected to the drain 128 of the charge accumulation device 104,a gate connected to the row line R3A, and a drain connected to thesecond column line C3B. The readout transistor M7 can have a gateconnected to the drain region 128 of the charge accumulation device 104(and thus also the source of the reset transistor M6), a sourceconnected to first column line C3A, and a drain connected to the secondcolumn line C3B. Turning on the reset transistor M6 can remove thecharge packets 216 accumulated at the drain 128 of, and thus reset, thecharge accumulation device 104, similarly to as discussed with respectto various embodiments above. With the reset transistor M6 turned off,the readout transistor M7 can provide an output voltage as a function ofthe charge packets 216 accumulated at the drain 128 of the chargeaccumulation device 104 operating in a source follower configuration, orprovide an output current as a function of the charge packets 216accumulated at the drain 128 of the charge accumulation device 104operating in a common source configuration.

The charge accumulation device 104 can also include more than twocontrol electrodes. FIG. 10 depicts an embodiment of an ion-sensingpixel 246 d having two control and readout transistors and four chargeaccumulation device electrodes, including three control electrodes and afloating gate. Although FIG. 10 depicts a simplified representation ofthe charge accumulation device 104 b, actual embodiments can includeother components of the charge accumulation device, such as componentsdepicted in FIG. 2. The electrodes of the charge accumulation device 104b can include a first and second electrode 186, 294 on one side of thefloating gate 184 and a third electrode 188 on the other side of thefloating gate 184. The charge accumulation device 104 b of FIG. 10 maynot include a source region, but can include a drain region 128 adjacentto the region under the third electrode 188. The control and readoutcircuit 112 is the same as that depicted in FIG. 9, and can include twotransistors, including a reset transistor M8 and a readout transistorM9, and receive a row line R4A and a plurality of column lines,including first and second column lines C4A, C4B. The reset transistorM8 can have a source connected to the drain 128 of the chargeaccumulation device 104 b, a gate connected to the row line R4A, and adrain connected to the second column line C4B. The readout transistor M9can have a gate connected to the drain region 128 of the chargeaccumulation device 104 b (and thus also the source of the resettransistor M8), a source connected to first column line C4A, and a drainconnected to the second column line C4B. The reset and readouttransistors M8, M9 can operate as they can in the embodiment of FIG. 9.

FIGS. 11A-11D depict an embodiment of the operation of the pixelembodiment 246 d of FIG. 10 to accumulate a plurality of charge packets216 as a function of the solution ion concentration and generate anoutput signal as a function of the accumulated charge packets 216. FIGS.11A-11D depict a plurality of complete cycles of an embodiment ofoperation of the charge accumulation device 104 b. The top of each ofFIGS. 11A-11D depicts a simplified partially cross-sectional, partiallyschematic representation of the charge-accumulation device 104 b andcontrol and readout transistors M8, M9 of FIG. 10. For convenience ofillustration and explanation, other components of thecharge-accumulation device 104 b and control and readout transistors M8,M9 are omitted from the depictions, although omitted components can bepresent in actual embodiments. In each figure, below the representationof the charge-accumulation device 104 b and control and readouttransistors M8, M9 are four diagrams depicting, in a manner spatiallyaligned to the pixel representation above, potential energies and chargeaccumulation in the charge accumulation device 104 b and control andreadout transistors M8, M9 at different phases during the operationalcycle of the pixel 246 d.

In a reset phase 298 depicted in FIG. 11A, a potential gradient can becreated from left to right under the first electrode 294, secondelectrode 186, floating gate 184 and third electrode 188, and a logichigh or high-voltage value delivered to the gate of the reset transistorM8 to clear any charge from the charge accumulation device 104 bremaining from previous operations.

In a first phase 302 of an operational cycle of the pixel 246 d, thepotential barrier under the third electrode 188 can then be raisedhigher than that in the reset phase 298 and that that existing on thefloating gate 184. In a second phase 306 of the pixel operational cycle,charge can be injected into the charge accumulation device 104 b throughthe drain diffusion node 128. In a third phase 310, the injected chargecan be withdrawn from the charge accumulation device 104 b, while acharge packet 216 remains under the floating gate 184 due to thepotential barrier differential between the floating gate 184 and thethird electrode 188 established in the second phase 306. In a fourth andfinal phase 314 of the operational cycle, a potential gradient can becreated from right to left under the third electrode 188, floating gate184, second electrode 186 and first electrode 186, to move the chargepacket 216 from under the floating gate 184 to under the first electrode294.

This accumulation cycle can then be repeated a plurality of times, asdepicted in FIGS. 11B-11C, to accumulate a plurality of charge packets216 under the first electrode 294.

FIG. 11D depicts a readout sequence to generate an output a signal basedon the size of the plurality charge packets 216 accumulated under thefirst electrode 294. The readout sequence can incorporate a doublesampling technique in which both a first output and a second output isprovided. The first output can represent an output produced by abackground level of charge present at the drain 128 of the chargeaccumulation device 104 b. The second output can represent an outputproduced by a both the background level of charge and the plurality ofcharge packets 216 accumulated at the first electrode 294 after theyhave been transferred to the drain 128 of the charge accumulation device104 b. Subtracting the first output from the second output can thusproduce a more accurate measurement of the charge packets 216representing the measurement of the solution ion concentration. In afirst phase 318 of the readout sequence, the first output can beprovided by the readout transistor M9 while the plurality of chargepackets 216 are still accumulated under the first electrode 294. In asecond phase 322 of the readout sequence, a potential gradient can becreated from left to right under the first electrode 294, secondelectrode 186, floating gate 184 and third electrode 188 to move theplurality of charge packets 216 from under the first electrode 294 tothe drain 128 of the charge accumulation device 104 b. After the secondphase 322, in a third phase 326 of the readout sequence, the secondoutput can be provided by the readout transistor M8.

FIG. 11D also depicts two phases 330, 334 of a deactivation sequence fora row of the pixel embodiments 246 d of FIG. 10.

Variations on the above discussed embodiments are possible. Althoughembodiments of the charge accumulation device have been depicted anddiscussed herein as including n-type sources and drains, and p-typeregions under the floating gate and control electrodes, in embodimentsthis relationship can also be reversed, with the charge accumulationdevice including p-type sources and drains, and n-type regions under thegate and control electrodes. Although FIG. 2 depicts an epitaxial p-typeregion under the control electrodes an floating gate, in embodiments ap-type or n-type region under the control electrodes and floating gatecan include a diffusion region or other type of p-type or n-type regioninstead of or in addition to an epitaxial region. Although variouscontrol and readout transistors have been discussed herein as includingNMOS transistors, in embodiments the control and readout transistors caninclude NMOS transistors, PMOS transistors, or any combination thereof.In some embodiments, rows and columns of pixels of the pixel array canshare some or all of the row and column lines. Although in someembodiments discussed herein, charge carriers and charge packets havebeen assumed to be electrons, in other embodiments charge carriers andcharge packets can comprise holes. Although stages of operational cyclesof the charge accumulation device have been discussed herein asoccurring in certain orders, and having certain labels (e.g., firstphase, second phase, etc.), in other embodiments, the labels and stagesof the operational cycles can be changed, rearranged, added to,subtracted to, etc. Although embodiments of the charge accumulationdevice have been discussed as having two or three control electrodes, inother embodiments, the charge accumulation device can have more or lessthan two or three control electrodes. Although certain embodiments ofcontrol signals have been depicted and discussed herein, in otherembodiments, the various control signals can take different forms.

Additional embodiments of the charge accumulation device 104 and ionsensitive pixel 246 are also possible. For example, any feature of anyof the embodiments of the charge accumulation device 104 and ionsensitive pixel 246 described herein can optionally be used in any otherembodiment of the charge accumulation device 104 and ion sensitive pixel246. Embodiments of the charge accumulation device 104 and ion sensitivepixel 246 can also optionally include any subset of the components orfeatures of any embodiments of the charge accumulation device 104 andion sensitive pixel 246 described herein.

Although the invention has been described above with reference tospecific embodiments, the invention is not limited to the aboveembodiments and the specific configurations shown in the drawings. Theoperation processes are also not limited to those shown in the examples.Those skilled in the art will appreciate that the invention may beimplemented in other ways without departing from the sprit andsubstantive features of the invention. The present embodiments aretherefore to be considered in all respects as illustrative and notrestrictive. The scope of the invention is indicated by the appendedclaims rather than by the foregoing description, and all changes thatcome within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

Several embodiments of the present invention are specificallyillustrated and described herein. However, it will be appreciated thatmodifications and variations of the present invention are covered by theabove teachings. In other instances, well-known operations, componentsand circuits have not been described in detail so as not to obscure theembodiments. It can be appreciated that the specific structural andfunctional details disclosed herein may be representative and do notnecessarily limit the scope of the embodiments.

Those skilled in the art may appreciate from the foregoing descriptionthat the present invention may be implemented in a variety of forms, andthat the various embodiments may be implemented alone or in combination.Therefore, while the embodiments of the present invention have beendescribed in connection with particular examples thereof, the true scopeof the embodiments and/or methods of the present invention should not beso limited since other modifications will become apparent to the skilledpractitioner upon a study of the drawings, specification, and followingclaims.

Various embodiments may be implemented using hardware elements, softwareelements, or a combination of both. Examples of hardware elements mayinclude processors, microprocessors, circuits, circuit elements (e.g.,transistors, resistors, capacitors, inductors, and so forth), integratedcircuits, application specific integrated circuits (ASIC), programmablelogic devices (PLD), digital signal processors (DSP), field programmablegate array (FPGA), logic gates, registers, semiconductor device, chips,microchips, chip sets, and so forth. Examples of software may includesoftware components, programs, applications, computer programs,application programs, system programs, machine programs, operatingsystem software, middleware, firmware, software modules, routines,subroutines, functions, methods, procedures, software interfaces,application program interfaces (API), instruction sets, computing code,computer code, code segments, computer code segments, words, values,symbols, or any combination thereof. Determining whether an embodimentis implemented using hardware elements and/or software elements may varyin accordance with any number of factors, such as desired computationalrate, power levels, heat tolerances, processing cycle budget, input datarates, output data rates, memory resources, data bus speeds and otherdesign or performance constraints.

Some embodiments may be implemented, for example, using acomputer-readable medium or article which may store an instruction or aset of instructions that, if executed by a machine, may cause themachine to perform a method and/or operations in accordance with theembodiments. Such a machine may include, for example, any suitableprocessing platform, computing platform, computing device, processingdevice, computing system, processing system, computer, processor, or thelike, and may be implemented using any suitable combination of hardwareand/or software. The computer-readable medium or article may include,for example, any suitable type of memory unit, memory device, memoryarticle, memory medium, storage device, storage article, storage mediumand/or storage unit, for example, memory, removable or non-removablemedia, erasable or non-erasable media, writeable or re-writeable media,digital or analog media, hard disk, floppy disk, Compact Disc Read OnlyMemory (CD-ROM), Compact Disc Recordable (CD-R), Compact DiscRewriteable (CD-RW), optical disk, magnetic media, magneto-opticalmedia, removable memory cards or disks, various types of DigitalVersatile Disc (DVD), a tape, a cassette, or the like. The instructionsmay include any suitable type of code, such as source code, compiledcode, interpreted code, executable code, static code, dynamic code,encrypted code, and the like, implemented using any suitable high-level,low-level, object-oriented, visual, compiled and/or interpretedprogramming language.

What is claimed is:
 1. (canceled)
 2. A method of detecting an ionconcentration of a fluid, comprising: passing the fluid having the ionconcentration over an ion-sensitive passivation surface above a gatestructure of a charge accumulation device of an ion sensitive circuit;individually forming, one at a time, a plurality of charge packets in agate semiconductor region of the charge accumulation device below thegate structure as a function of the ion concentration in the fluid;applying a control signal to a control electrode above an electrodesemiconductor region of the charge accumulation device to controltransmission of the charge packets one at a time from the gatesemiconductor region; accumulating the plurality of charge packets at adrain diffusion region of the charge accumulation device at a selectablecharge packet accumulation frequency; and generating an output signalusing at least one control and readout transistor as a function of theaccumulated plurality of charge packets at the drain region at aselectable output signal generation frequency, wherein the output signalrepresents a measure of the ion concentration in the fluid.
 3. Themethod of claim 2, further comprising individually transmitting, one ata time, the plurality of charge packets through the electrodesemiconductor region in response to the applying the control signal tothe control electrode.
 4. The method of claim 2, further comprisinginjecting charge into the gate semiconductor region from a source regionthrough a second control electrode region to provide charge to form thecharge packet.
 5. The method of claim 2, further comprising removing anycharge packets previously accumulated at the drain diffusion regionprior to accumulating the plurality of charge packets.
 6. The method ofclaim 2, wherein the output signal generation frequency is less than thecharge packet accumulation frequency.